Generally, lubricating oils include base oils and additives for improving the physical properties of the base oils, and the base oils are typically classified to mineral oils and synthetic oils. In general, the mineral oils indicate naphthenic oils which are produced during the separation and purification of crude oils, and the synthetic oils indicate poly-α-olefin (PAO)s which are manufactured by polymerization of α-olefin produced during an oil refining process. In the past, mineral oils had been mainly used as the lubricating base oils, however in recent years, the demand for the synthetic oils having the characteristics of low temperature fluidity, high viscosity index, low volatility at a high temperature, high shear and thermal stability and the like, is increasing. The synthetic oils, compared with the mineral oils, have small viscosity variation according to temperature variation so that excellent lubricity is maintained regardless of seasonal change. Therefore, the synthetic oils have contributed to quietness and fuel efficiency improvement of vehicles, and also, they have some advantages of excellent durability and stability, long lifespan, and being environmentally friendly because of generating less sludge and waste oils. Engine oils using conventional mineral oils do not have sufficient physical and mechanical properties required for the recent engines which are downsized and optimized for high efficiency. Accordingly, the demand for the synthetic oils is increasing in the field of engine oils, gear oils, grease and so on, requiring high quality.
The poly-α-olefins (POAs) used as the synthetic oils are disclosed in U.S. Pat. No. 3,780,128, in which the POAs can be obtained by oligomerization of higher linear α-olefins such as decene-1(C10), dodecene-1(C12) and so on, in the presence of an acid catalyst. However, it has drawbacks that raw materials of higher linear alpha-olefin (LAO) are expensive and supply thereof is not stable. On the other hands, Japanese Unexamined-Publication 1982-117595 discloses a preparing method for synthetic lubricating oils by copolymerizing the ethylene and alpha-olefin, the synthetic lubricating oils having excellent properties in view of viscosity index, oxidation stability, shear stability, heat resistance etc. In the copolymerization of ethylene and alpha-olefin, is used a catalyst composition composed by titanium compound and organic aluminum compound. The titanium compound catalyst has large catalytic activity, but molecular weight of the copolymer obtained has wide distribution and low regularity. Accordingly, it is difficult to obtain products having high flash points which are useful for lubricating oils, lubricating oil additives, fuel oil additives etc., and in case of high viscosity products, cost thereof is high so that it is not practical. Also, U.S. Pat. No. 5,767,331 discloses a method for copolymerizing the ethylene and alpha-olefin, specifically copolymerizing ethylene and propylene, by using vanadium-based catalyst composition containing vanadium compound and organic aluminum compound. The copolymer prepared by using vanadium-based catalyst composition has narrow molecular weight distribution and superior uniformity. But the copolymer prepared by using vanadium-based catalyst composition generally has very low polymerization activity, and accompanies large amount of catalyst sludge so that it has a drawback of requiring the additional de-catalytic process, which is common problem on the 1st generation catalyst such as Ziegler-Natta catalyst. In addition, Japanese Unexamined-Publication S61-221207, Japanese Unexamined-Publication H7-121969 etc. disclose a method for preparing the copolymer with high polymerization activity by using a catalyst system composed by metallocene compound such as zirconocene and so on and organoaluminum oxy-compound, and Japanese Patent 2796376 discloses a method for preparing synthetic lubricating oils by copolymerizing the ethylene and alpha-olefin, by using a catalyst system composed by specific metallocene catalyst and organoaluminum oxy-compound.